1. Field of the Invention
The present invention is related to fuel cell bipolar plates with improved conductivity and hydrophilicity and methods for making such plates.
2. Background Art
Fuel cells represent a clean alternative to current technologies using fossil fuel resources. Polymer electrolyte membrane (“PEM”) fuel cells have gained prominence and are found in a wide range of applications due to their high power density, quick start-up and compatibility with automotive applications.
PEM fuel cells typically have a solid polymer membrane with anode and cathode catalytic layers deposited on the opposite faces of the membrane. The combination of catalytic mixtures and the membrane defines a membrane electrode assembly (“MEA”). MEA's are relatively expensive to manufacture and require certain conditions for effective operation. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. The electrodes in the MEA must be chemically inert, electrically conductive and stable. Typically, the electrodes are made of porous finely divided catalytic particles (for example, platinum) supported on carbon particles and mixed with an ionomer. In a typical operation of a fuel cell, at the anode, a platinum-containing catalyst stimulates dissociation of the fuel to hydrogen protons and electrons. The electrons migrate to the cathode via an external circuit and create an electrical current. The hydrogen protons migrate through the membrane to the cathode completing the overall reaction. At the same time, oxygen in pure form (O2) or air (a mixture of O2 and N2) is being fed to the cathode, where a catalyst stimulates formation of oxygen ions that react with hydrogen protons, creating water and heat as byproducts.
Another component of the PEM cell is a gas diffusion layer (“GDL”). There is one GDL on the side of each electrode, made of a porous, electrically conductive and gas impermeable material (usually wet-proofed carbon cloth or carbon paper). The GDL provides electrical contact between the electrodes and the bipolar plates. The porous nature of the GDL material ensures effective and uniform access of the fuel and oxidant to the surface of the catalyzed membrane. The GDL also assists in water management of the cell by allowing the appropriate amount of water vapor to reach the MEA, thus preventing loss of ionic conduction by keeping the membrane humidified.
Several fuel cells are usually combined in a fuel cell stack to generate the desired power, and fuel stacks can be arranged to form a multi-stack array. Such a fuel cell stack includes a series of bipolar plates, also known as flow field plates, positioned between adjacent fuel cells in the stack. The MEA/GDL assembly is sealed between a pair of bipolar plates, which typically have several important functions: (1) to distribute the fuel and oxidant within the cell using flow channels on both sides, (2) to facilitate water management within the cell, (3) to separate the individual cells in the stack and act as a support structure (4) to carry electrical current away from the interior to the exterior of each cell, (5) to conduct heat away from the cell. Plate topologies (such as surface features), materials and coatings facilitate these functions and protect the plates from a hostile operating environment of very low pH and high temperatures. There are several alternative conductive materials for the bipolar plates, such as non-porous graphite, stainless steel, aluminum, and metal- or carbon-based composites. Metal bipolar plates typically produce a natural oxide on their outer surface that makes them resistant to corrosion. However, this oxide layer is not conductive, and thus increases the internal resistance of the fuel cell, reducing its electrical performance.
Bipolar plates represent a significant portion of fuel cell cost and constitute the dominant weight of the cell stack. The potential benefits of improved efficiency of the system enable the formation of smaller stacks, decrease in packaging requirements and lowering costs, which lead to increased applicability of PEM fuel cells in the automotive industry.
During the operation of the fuel cell, especially at low power demands, moisture in the form of droplets accumulates within the flow channels of the bipolar plate. Droplets continue to expand due to the typically hydrophobic nature of the channel surface and block the passage of the reactant gases through the flow channels. Thus, areas of the membrane deprived of the reactant gas will not generate electricity resulting in a reduction of the overall efficiency of the fuel cell to the point of the cell failure. Typically, the problem might be somewhat alleviated by purging the water from the flow channels using hydrogen gas or by reducing inlet reactant gas humidification. Both of these solutions have disadvantages, such as a reduction in fuel economy and in long-term durability of the membrane. As is well understood in the art, hydrophilic coating may be applied to the surface of the flow channel to improve channel water transport. A hydrophilic coating causes water to spread along its surface in a process of spontaneous wetting and form a thin film. The thin film tends to cause less obstruction of the gas flow in the channel system and eventually water is transported along the channel toward the outlet by capillary forces.
Typical conductive coatings employed to reduce the contact resistance on a plate surface include relatively costly materials such as gold and its alloys, and composite polymeric materials, which require costly production equipment. Current hydrophilic coatings are mainly nanoparticulate silicas, or organic-based particles. However, these coatings are unstable over time, do not adhere well to the substrate material of the plate and are expensive to manufacture.
Accordingly, there is a need for improved methodology for producing coatings that combine excellent mechanical properties with enhanced electrical conductivity and hydrophilicity.